Climate change deniers are "ridiculous" and akin to "flat-earthers", according to Sir Nicholas Stern, who advised the government about the economic threat posed by global warming. The respected economist compared climate naysayers to those who deny the link between smoking and cancer or HIV and Aids in the face of mounting scientific evidence.

Stern — who prepared his influential report to the UK Treasury in 2007 at Gordon Brown's request — said the evidence that human-induced climate change was occurring was "crystal clear".

"If you look at all the serious scientists in the world, there is no big disagreement on the basics of this ... it would be absolute lunacy to act as if climate change is not occurring," he said.

The US-based Heartland Institute, which had been funded by Exxon Mobil until 2006, launched its gathering of more than 70 participants in an event entitled 'Global warming: Was it ever really a crisis?'

Lord Stern addressed a large conference in London yesterday, organised by the Department for International Development, where he said the battle against poverty and the management of climate change were the "two great challenges of the 21st century ".

"We know that greenhouse gases are rising. And we know the [global] temperature is rising. We can look back through ice-core data and see over 800,000 years, relationships between carbon dioxide and the temperature of the world," he told the Guardian.

"So those people who deny the importance of climate change are just wasting their time. They're also being diversionary because if we don't act the risks are enormous."

Asked if the public debate on the issue was being won, he said: "Those who say that climate change doesn't exist are being understood as the flat-earthers that they are, as the people who deny the link between smoking and cancer, as the people who denied the link between HIV and AIDS.

"They are marginal and they are ridiculous. And they are very confused."

He admitted that while some time still had to be spent dealing with "the silly arguments that [deniers] put", discussions around the world had moved on and were focused on the details of "reducing emissions and managing the impact of climate change".

His comments will be bolstered this week by President Barack Obama's directive that American government agencies should pick advisers based on expertise, not political ideology. The move was hailed by scientists who felt the previous administration had filled agencies with supporters who shared George Bush's scepticism about climate change.